associated types and bounds

[sineptic]

It's (I think) have save root of problem like in my previous issue #2771.

first try

trait SomeTrait<'a>: Serialize + Deserialize<'a> {
    type SomeType: Serialize + Deserialize<'a>;
    // ...
}

#[derive(Serialize, Deserialize)]
// I must use it because I don't have access to 'de lifetime
#[serde(bound(deserialize = "T: SomeTrait<'de>"))]
struct SomeStruct<T> {
    a: T,
    field2: T::SomeType,
}

COMPILE ERROR!!!: associated type SomeType not found for T

---

second try

#[derive(Serialize, Deserialize)]
#[serde(bound(deserialize = "T: SomeTrait<'de>"))]
struct SomeStruct<T> {
    a: T,
    // COMPILE ERROR!!!: associated type `SomeType` not found for `T`
    field2: T::SomeType,
}

... some other errors,
help: use fully-qualified syntax to disambiguate
|
14 | field2: <T as SomeTrait<'a>>::SomeType,
| ~~~~~~~~~~~~~~~~~~~~~~
help: use fully-qualified syntax to disambiguate
|
14 | field2: <T as SomeTrait<'de>>::SomeType,
| ~~~~~~~~~~~~~~~~~~~~~~~

hmm.. compiler see <T as SomeTrait<'de>>::SomeType?

---

third try

#[derive(Serialize, Deserialize)]
#[serde(bound(deserialize = "T: SomeTrait<'de>"))]
struct SomeStruct<T> {
    a: T,
    // COMPILE ERROR!!!: associated type `SomeType` not found for `T`
    field2: <T as SomeTrait<'de>>::SomeType,
}

aghh...
14 | field2: <T as SomeTrait<'de>>::SomeType,
| ^^^ undeclared lifetime

---

Ok, I know how to make it work:

#[derive(Serialize, Deserialize)]
#[serde(bound(deserialize = "'a: 'de, 'de: 'a"))]
struct SomeStruct<'a, T: SomeTrait<'a>> {
    a: T,
    field2: T::SomeType,
}

BUT

1. It looks ugly.
2. Now I must in every impl block add 'a twice.